This invention relates to printing presses, and more particularly, to an anti-marking method and apparatus for preventing the marking and marring of printed sheets in a high speed, sheet-fed multicolor rotary printing press of the type having a perfecting cylinder for permitting either one sided or two sided printing.
Many high speed, sheet-fed rotary printing presses are provided with perfecting cylinders which permit the press to print on two sides of a sheet, and which can also be used for single sided, multi-color printing. Examples of such presses are those currently manufactured by Heidelberger Druckmaschinen AG, of Heidelberg, West Germany under its designations "Speedmaster", "M-Offset" and "GTO". In such presses, a perfecting cylinder is used to transfer sheets which have been printed on one side in a first printing station, from a storage drum to a second printing station for further printing. The perfecting cylinder is adjustably timed with respect to the storage drum such that the perfecting cylinder either transfers the sheet, leading edge first, to the second printing station where a second color ink is applied to the same side previously printed (referred to as one sided or non-perfector mode printing), or the sheet is reversed and transferred to the second printing station trailing or tail edge first for printing on the reverse side (referred to as perfector mode printing).
When used in the perfector mode for two sided printing, sheet grippers carried by the perfecting cylinder which project radially from a longitudinal opening formed along the length of the cylinder, are timed to grip the tail edge of the sheet from the storage drum after the leading edge of the sheet on the storage drum has passed through the nip between the perfecting cylinder and the storage drum, and to pull the sheet from the storage drum with the non-printed side supported by the surface of the perfecting cylinder. When used in the non-perfector mode, the leading edge of the sheet is gripped by the grippers of the perfecting cylinder at the nip, and pulled from the storage drum with the wet inked side facing and supported by the surface of the perfecting cylinder.
The storage drum typically is sized to have a diameter twice that of the perfecting cylinder, and carries two sets of diametrically opposed sheet gripper mechanisms, each set comprising sheet grippers for the leading edge of the sheet and sheet grippers for the tail edge of the sheet. Since the perfecting cylinder must allow the leading edge of the sheet carried by the storage drum to pass through the nip when the press is used in the perfector mode, the supporting surface of the perfecting cylinder is provided with a series of longitudinally spaced circumferential channels or grooves which allow the leading edge sheet grippers of the storage drum to pass through the nip. When the press is used in the non-perfector mode, however, since the leading edge of the sheet is transferred to the perfecting cylinder at the nip, grooves in the supporting surface of the perfecting cylinder are not required since the leading edge grippers of the storage drum essentially mate with the opening in the perfecting cylinder from which its sheet grippers project.
During perfector mode printing, the perfecting cylinder operates to reverse the sheet so that the non-inked side of the sheet is carried by the supporting surface of the perfecting cylinder, and marking and marring of the freshly printed sheet can not occur. However, marking and marring of the freshly printed sheet has been found to occur when the press is used in the non-perfector mode since the perfecting cylinder merely acts as a conventional press transfer cylinder with the printed side of the sheet carried against the supporting surface of the perfecting cylinder during the transfer and the grooved surface does not provide uniform sheet support.
It is now well recognized and accepted in the printing industry that marking and marring of freshly printed sheets caused by engagement of the wet inked surface with the supporting surface of a conventional press transfer cylinder can be substantially eliminated by using an anti-marking net system such as disclosed in the DeMoore U.S. Pat. No. 4,402,267 issued Sept. 6, 1983 and entitled "Method and Apparatus for Handling Printed Sheet Material", the disclosure of which is incorporated herein by this reference. That system, which is marketed under license by Printing Research, Inc. of Dallas, TX under the registered trademark "Super Blue", includes the use of a low friction coating on the supporting surface of the transfer cylinder, and over which is loosely disposed a fabric cover referred to in the trade as a "net". It is believed that the loosely mounted net attaches and clings to the wet inked side of the freshly printed sheet as the sheet is supported by the transfer cylinder such that any relative motion between the sheet and the cylinder takes place between the surface of the net and the low friction coated surface of the cylinder so that marking and marring of the freshly printed surface does not occur.
While attempts have heretofore been made to adapt the anti-marking system disclosed in the DeMoore Pat. No. 4,402,267 for perfecting cylinders, prior to the present invention none had proved satisfactory. One such attempt has been to simply mount a net over the grooved supporting surface of the perfecting cylinder when ever the press is to be used in the non-perfector mode. This approach, however, has not proved satisfactory since the grooved surface of the cylinder does not provide a smooth, uniform support surface for the sheet, and it has been found that marking and marring of the sheet in the area of the unsupported portions of the net overlying the grooves may occur. Another suggestion has been to replace the perfecting cylinder with a smooth surfaced cylinder such as a conventional transfer cylinder, but that approach is impractical and destroys the purpose of the perfecting cylinder since the press can not then be used in the perfector mode.
Thus, there exists a need for an effective and reliable method and apparatus to prevent freshly printed sheets from being marked and marred by the grooved supporting surface of the perfecting cylinder when the press is operated in the non-perfector mode, yet which is relatively simple in design, inexpensive to manufacture and can be quickly and easily installed or removed from the press with a minimum of lost press production time during conversion between perfector and non-perfector modes. As will become apparent from the following, the present invention satisfies this need in a novel and unobvious manner.